Circuit breakers and ground-fault circuit interrupters (GFCIs) are generally well known in the art. A GFCI prevents shock or electrocution from a ground-fault, which is an unintended conductive path between an ungrounded current carrying conductor and earth ground that can occur, for example, when a plugged-in electrical appliance is dropped into a sink, pool, pond, puddle or hot tub. The GFCI cuts off power to the downstream circuits when it detects an imbalance in the load current (i.e., outgoing current different from returning current) that exceeds the allowable amount of ground fault leakage current. The leakage current path to ground may include the grounded metal case of an appliance or a person in some instances.
GFCIs can also protect against grounded-neutral faults. Unlike ground-faults, a grounded-neutral fault is a low impedance path between a ground wire and a neutral wire and occurs in situations where the neutral conductor is touching the ground conductor used for grounding equipment enclosures and structures. Ground fault current carried on the ground conductor has a return path that splits between the ground conductor and the neutral conductor. The ground fault current returning through the neutral conductor cancels out part of the existing ground fault signal, which desensitizes the ground fault detection circuit and can result in dangerously high ground fault currents appearing in the leakage path.
Presently available GFCI technology frequently uses a sense circuit having a differential current transformer and a single ADC to monitor for both ground-faults and grounded-neutral faults. Ground-faults are often detected by comparing the current in the sense circuit on the secondary side of the transformer to a reference value during a first time interval using the ADC. Grounded-neutral faults are often detected by monitoring the shorted primary circuit impedance reflected back on to the secondary of the transformer. One method for monitoring for grounded-neutral faults is by injecting current into the sense circuit during a second time interval to produce a decaying sinusoidal signal in the sense circuit. The amplitude of the decaying sinusoidal signal is measured during this interval by the ADC and the measured amplitude may then be used to determine the presence of grounded-neutral faults.
Recently, arc-fault circuit interrupters (AFCIs) have begun to be required in new home and building constructions. Arc-faults are intermittent faults that can be caused, for instance, by worn or damaged insulation, loose connections, broken conductors, and the like. Because of their intermittence, arc-faults do not generate sustained currents of sufficient magnitude to readily trip a conventional thermal-magnetic circuit interrupter. Inputs such as band-pass filters, line current sensors, and voltage sensors must be sampled at regular time periods in order to implement arc-fault detection algorithms.
Attempts to integrate arc-fault protection and ground fault protection in a dual-function ground fault and arc-fault circuit breaker device have met with mixed results. Note that the term “ground fault” (without a hyphen) as used in the phrase “ground fault protection” or “dual-function ground fault and arc-fault protection” may refer to both a “ground-fault” and a “grounded-neutral fault.” In general, ground-fault sampling and grounded-neutral fault sampling occur during separate portions of a fault detection interval, while arc-fault sampling occurs throughout the fault detection interval. This can create a conflict because arc-fault sampling and grounded-neutral fault sampling use some of the same resources.
Specifically, grounded-neutral fault detection requires the ADC to frequently sample the amplitude of the decaying sinusoidal signal during a portion of the fault detection interval. However, because the ADC is regularly dedicated for sampling arc-fault related inputs and signals, a resource conflict may arise with respect to the ADC during this portion of the detection interval. As a result, it has heretofore not been practical to perform both arc-fault detection and grounded-neutral fault detection concurrently using a single ADC due to the potential for mutual interference. Adding an additional ADC, on the other hand, would increase the overall complexity and cost of the circuit breaker device.
Accordingly, what is needed is an efficient and cost-effective way to integrate arc-fault detection and ground fault detection in a single circuit breaker device. More specifically, what is needed is a way to perform arc-fault sampling and grounded-neutral fault sampling in the same sampling interval using a single ADC without creating mutual interference.